The invention relates to a method for processing workpieces by removing material.
Such methods serve e.g. for generating the paths of tools having five axes in CAD (Computer Aided Design)/CAM (Computer Aided Manufacturing) systems for the processing of workpieces with arbitrary surfaces.
All known methods for tool fitting start from from predefined contact paths of the tool on the workpiece. The position and orientation or the inclination angle (angle between the axis of the tool in the direction of movement of the tool and a vector normal to the desired surface) and the tilting angle (angle of the axis of the tool in the direction perpendicular to the direction of movement) are determined in different ways for selected points on the contact path. The presently best method (Jean-Pierr Kruth and Paul Klewais, Optimization and Dynamic Adaptation of the Cutter Inclination during Five-Axis Milling of Sculptured Surfaces, Annals of the CIRP, 1994) is based, for the determination of inclination and tilt angle, on projections of quadratic approximations of the workpiece and the tool. A quadratic equation for calculating the critical inclination angle (inclination angle where there is no more undercut at the approximations) is derived for constant tilt angles. In these methods the processing remains also still complicated in spite of the limited accuracy (danger of collision)
Hence, the problem to be solved is to provide a method of the type mentioned initially that avoids at least partially the disadvantages of known methods. In particular, a fast and therefore cheap processing should become possible.
This problem is solved with the object of the invention. In general, a tool is led along paths (B) over the workpiece, whereby material getting into a cutting range (xcfx84) of the tool is removed for generating a desired surface ("psgr"). The workpiece is positioned along its path by repetitively adjusting at least an inclination and a tilting angle of the tool in such a way that a magnitude of the area of tolerance is substantially maximum, where the area of tolerance is a continuous area within which the distance between the desired surface and the cutting range of the tool lies within a given range of acceptable mismatch.
In contrast to know methods, no local fitting in the area of the point of contact but a non-local optimization of a quantity, e.g. the width of the area of tolerance, is carried out, whereby the number of machining paths can be reduced and processing can be rationalized.
Preferably, the tilt and inclination angles are chosen such that the tool always remains above the desired surface. This allows a better manual finishing of the surface for adapting it to the desired shape. For the known method by Kruth and Klewais it can be shown using the exact descriptions of tool and workpiece surfaces that the tool can damage the workpiece (in particular when using large tools).
In an other preferred embodiment the tool paths are chosen such that for each point on the desired surface the area of tolerance and the direction of the maximum diameter of the area of tolerance are determined. The tool paths are chosen substantially perpendicular to this diameter, whereby the number of tool paths can be reduced.
In a further embodiment of the invention a fitting curve is determined for a point on the desired surface, which corresponds in its derivatives, in particular in its curvature, derivative of curvature and torsion at the given point, to a cutting curve that describes the cutting range of the tool (formed by all points that have a minimum distance to the desired surface). As shown in the following, such a method allows to determine a well adapted position of the tool in computationally simple manner.
Examples for processing methods where the present method can be used are five axis milling, grinding, erosion, lathing. Examples for workpieces are outer skin sections for airplanes, cars or ships, flow guiding pieces such as turbine blades, designer items, etc.